Vapour pressure and mixing thermodynamic properties of the KNbO₃-NaNbO₃ system

Equilibrium vapour pressures of sodium and potassium over a K_xNa_1-xNbO₃ solid solution within its whole compositional range at temperatures between 1173 K and 1303 K were determined by Knudsen Effusion Mass Spectrometry. It should be noted that the thermodynamic equilibrium between the condensed and the vapour phase could be established only after prolonged annealing (more than 10 h at 1263 K). The equilibrium vapour pressure of potassium over K_0.5Na_0.5NbO₃ (KNN) is a few times larger than that of sodium, i.e., 8 × 10^-3 Pa as compared to 3 × 10^-3 Pa at 1263 K. From the obtained results, the excess thermodynamic functions for the pseudo-binary KNbO₃-NaNbO₃ system were evaluated. The excess Gibbs energy was found to be positive, the excess enthalpy is close to zero, while the negative excess entropy indicates a partial ordering of alkaline ions in the solid solution. The comparison of the obtained results to the well-established lead-based piezoelectric systems revealed, that the vapour pressure of alkalis over the respective niobates at 1200 K is almost three orders of magnitude lower as compared to the values reported for lead oxide over Pb(Zr,Ti)O₃.